1. Field of the Invention
The invention relates to a telecommunication system, in particular a transparent satellite telecommunication system.
2. Description of the Prior Art
It is known in the art that diverse types of communication resources, in particular frequencies, time slots and codes, can be used to transmit a multiplicity of digital calls simultaneously.
Multiple frequency time division multiple access (MF-TDMA) transmission, which uses a multiplicity of frequencies and time slots simultaneously, provides a high data bit rate. This is why it is used in the DVB-RCS standard, currently in preparation, which concerns the transmission via satellite of digital data (audio, video, Internet, etc.).
In the above type of telecommunication system, each carrier, which has a particular frequency, offers a given number of time slots per frame, the number of time slots per frame being equal to 1 or to a power of 2, for example.
Messages are transmitted in superframes containing a particular number of frames. A frame, which comprises a particular number of carriers and time slots, is transmitted during a particular time and a superframe extends over a number N of frame durations and includes a number N1 of carriers.
During a frame, the same message may be transmitted on several different types of carrier, that is to say with time slots of different duration.
As a general rule, MF-TDMA calls can be “star” calls, i.e. calls made through the intermediary of a base station. In other words, a terminal communicates with another terminal via the base station. It is also possible to effect calls directly between terminals, without passing through the base station, and this type of call is called a “meshed” call. It is also possible to combine star calls and meshed calls.
It has been found that in the above type of telecommunication system the frequency hops for the same terminal can have large values, which makes the implementation of the terminals of the above kind of telecommunication system relatively complex.
An object of the invention is to enable total (star and meshed connection) connectivity using terminals having limited performance in terms of send and receive frequency hopping and using the MF-TDMA technique to send and receive.
The invention therefore calls for terminals equipped with an MF-TDMA modulator for star sending (to a base station) or meshed sending (to other terminals) and an MF-TDMA demodulator for meshed reception (from other terminals).
The frequency band allocated to the system is divided into two parts and the terminals are divided into two groups; the first group can send in the first part of the frequency band and the second group can send in the second part of the frequency band; each part of the frequency band is divided into two sub-bands, of which one is for star sending and the other is for meshed sending, the sub-bands for meshed sending being adjacent and the combination of the two sub-bands for meshed sending also being used for meshed receiving.
Accordingly, in each group of terminals, the send frequency hops are limited to the part of the band allocated to that group and the receive frequency hops are limited to the two adjacent central sub-bands.
The two parts of the band are preferably equal. Similarly, it is preferable if the central sub-bands allocated to meshed reception are both the same width.
However, the two sub-bands of each part can be unequal. For example, the width of the sub-band allocated to star sending can be significantly greater than the width of the sub-band allocated to meshed sending.
In one embodiment, the width of the combination of the two central sub-bands is less than the width of each of the sub-bands allocated to star sending.
As a general rule, the system's total star and meshed send band corresponds to at least twice the send band of a terminal and the total meshed receive band corresponds to the receive band of a terminal.
The invention relates not only to the telecommunication system as a whole, but also to its various component parts, in particular a base station and each of the terminals including an MF-TDMA modulator and an MF-TDMA demodulator.